Shonisaurus is a genus of ichthyosaur. At least 37 incomplete fossil specimens of the marine reptile have been found in the Luning Formation of Nevada, USA. This formation dates to the late Carnian age of the late Triassic period, about 215 million years ago.[1]

Shonisaurus lived during the Norian stage of the late Triassic period. S. popularis measured around 15 metres (49 ft) long. A second species from British Columbia was named Shonisaurus sikanniensis in 2004. S. sikkanniensis was one of the largest marine reptiles of all time, measuring 21 metres (69 ft). However, phylogenetic studies later showed S. sikanniensis to be a species of Shastasaurus rather than Shonisaurus.[2] A new study, of the year 2013, reassert the original classification, finding it more closely related to Shonisaurus than to Shastasaurus.[3] Specimens belonging to S. sikanniensis have been found in the Pardonet FormationBritish Columbia, dating to the middle Norian age (about 210 million years ago).[1]

Shonisaurus had a long snout, and its flippers were much longer and narrower than in other ichthyosaurs. While Shonisaurus was initially reported to have had socketed teeth (rather than teeth set in a groove as in more advanced forms), these were present only at the jaw tips, and only in the very smallest, juvenile specimens. All of these features suggest that Shonisaurus may be a relatively specialised offshoot of the main ichthyosaur evolutionary line.[4] It was historically depicted with a rather rotund body, but studies of its body shape since the early 1990s have shown that the body was much more slender than traditionally thought.[5]S. popularis had a relatively deep body compared with related marine reptiles.[1]

Shonisaurus was also traditionally depicted with a dorsal fin, a feature found in more advanced ichthyosaurs. However, other shastasaurids likely lacked dorsal fins, and there is no evidence to support the presence of such a fin in Shonisaurus. The upper fluke of the tail was probably also much less developed than flukes found in later species.[6]

Fossils of Shonisaurus were first found in a large deposit in Nevada in 1920. Thirty years later, they were excavated, uncovering the remains of 37 very large ichthyosaurs. These were named Shonisaurus, which means "lizard from the Shoshone Mountains", after the formation where the fossils were found.

The Nevada bonebed represents a large assemblage of Shonisaurus which died at varying times and became preserved on the sea floor in a curiously regular arrangement of bones. The lack of fossil invertebrates encrusting the remains indicates that the carcasses sank in relatively deep water poor in oxygen.

In a 2011 lecture to the Geological Society of America, Mark McMenamin and Dianna Schulte McMenamin, geologists from Mount Holyoke College, put forward the controversial hypothesis that the large assemblage of remains were placed in deep water by an unidentified, gigantic, squid-like predator they referred to as a "kraken". The McMenamins went on to suggest that the arrangement of the vertebrae, which appear to resemble cephalopod suction organs of cephalopod tentacles in their round, concave shape, were purposefully arranged by a Triassic kraken (which they suggested would have had to be extremely intelligent) in order to create a work of art, specifically a self-portrait of its own tentacles.[8] This hypothesis was widely reported by the news media when the Geological Society of America issued a press release in October 2011. Critics were concerned that there was only trace fossil evidence favoring the existence of the Triassic kraken. Science author Brian Switek, writing for the Wired magazine's Laelaps web log, made the following comment:

"[T]he fact remains that journalists should have actually done their jobs rather than act as facilitators of hype. You don’t have to be a paleontologist to realize that there’s something fishy about claims that there was a giant, ichthyosaur-crunching squid when there is no body to be seen."[9]

Other writers responded more favorably to the McMenamins' proposal. Writing for About.com, Andrew Alden wrote that the concept of high intelligence in the Triassic kraken hypothesis "is its most valuable contribution: why should we rule out intelligence in the distant past? And what would the signs of it be?"[10]